Alternating current motors are widely used for a wide variety of different purposes. Such motors range in size from very small fractional horsepower motors on up to multiple horsepower sizes. Most large horsepower electric motors are three phase motors, while the majority of smaller fractional horsepower and low horsepower motors are operated from a source of single phase alternating current electricity. Single phase motors are particularly popular since most home and business alternating current supplies are in the form of single phase supplies.
Electric motors typically include a stator, which is wound with start windings and run windings connected to the source of operating power. The stator windings surround a rotor which rotates a shaft to produce the motor output. Rotors are made in a number of different configurations, such as squirrel cage rotors, high resistance rotors, low resistance rotors, wound rotors or multiple winding high and low resistance rotors. All of these configurations, along with various stator win ings arrangements, are well known in the electric motor industry.
Conventional single phase motors frequently are constructed in a "capacitor-start" or "capacitor-start/capacitor-run" configuration. In capacitor-start motors, the start winding is connected in series with a starting capacitor and a centrifugal or thermal switch across the input terminals. The run winding is connected in parallel with this series-connected starting circuit. In such capacitor start motors, the starting condition is such that the instantaneous locked rotor current is high and the motor starting current demand factor also is high. Such motors undergo relatively high operating temperatures and require some type of mechanical switch (such as a centrifugal or thermal switch) for disconnecting or opening the starting winding circuit after a preestablished rotational speed of the rotor is reached. The starting winding in such capacitor start motors typically is wound with relatively fine or small diameter wire compared to the wire used in the run winding. Because small gauge fine wire is used in the starting winding, such motors have a relatively limited life due to burn-out of the start winding.
In addition, the requirement for the cut-out switch in series with the start winding and start capacitor results in additional complexity, cost, and potential for failure of such motors. For example, if a centrifugal switch is employed, the switch contacts necessarily must be built into the circuit, as well as the interconnections of the mechanical centrifugal switch parts with the shaft of the rotor. Such parts are subject to failure and in fact do fail.
It is desirable to provide an electric motor operated from single phase operating current power which employs start windings and run windings electrically displaced in the stator by 90.degree. without requiring cut-out switches, centrifugal switch, relays or other devices to disconnect the starting winding upon the motor reaching its running condition of operation.